Wide gap semiconductor materials such as silicon carbide (hereinafter represented as SiC) have excellent physical characteristics when compared with silicon (hereinafter represented as Si), that is, those materials have larger energy gaps, higher breakdown field strength by about one digit, and so on. Because of this, SiC receives attention as a suitable semiconductor material used for power semiconductor devices having high heat resistance and high withstand voltage.
As an example of conventional power semiconductor devices with high heat resistance and high withstand voltage using SiC, a power semiconductor device using a SiC diode element shown below is disclosed in Proceedings of 2001 International Symposium on Power Semiconductor Devices & IC's, pp. 27 to 30 (see Prior Art 1).
In the SiC diode element, a pn junction, which injects electrical charge on an SiC substrate, is formed of an epitaxial film grown by an epitaxial growth technique. The epitaxial film on the end region of the substrate is removed by mesa etching, following which a termination portion, which relieves an electric field, is formed by ion implantation. Specifically, the p-type epitaxial layer 0.7 μm thick is removed by using a mesa etching process of about 1 μm in depth, after which a passivation film is formed by using a 0.4 μm thick inorganic film such as silicon dioxide film. This conventional SiC diode element has a high withstand voltage of 12 kV to 19 kV.
FIG. 6 is a cross-sectional view of a package in which an SiC diode device is formed by using the conventional SiC diode element described above. In FIG. 6, an SiC diode element 90 is mounted on the upper surface of a metal support 93, which has a cathode terminal 92 on its lower surface, in such a way that the cathode electrode 97 of the diode element 90 is connected to the upper surface of the support 93. The support 93 is further provided with an anode terminal 91 in such a manner that the terminal 91 passes through the support 93 with electrical insulation kept between them via an insulator 12. The anode terminal 91 is connected to the anode electrode 96 of the SiC diode element 90 by using a lead wire 8. A metal cap 94 is provided on the upper surface of the support 93 so as to cover the diode element 90, thereby a space including the diode element 90 is sealed. The space 95 is filled with sulfur hexafluoride gas. The reason why sulfur hexafluoride gas is provided is as follows: since a creeping distance between the anode electrode 96 and the exposed sides 90a uncovered with a passivation film 98 is short, its withstand voltage cannot be increased.
To increase the withstand voltage, it is also considered that inert gasses such as nitrogen gas and noble gasses such as argon gas are used as insulating gas. However, since these gasses are low in maximum breakdown electric field strength, electric discharge develops in the gasses during the application of high voltages, thereby the SiC diode element 90 itself and passivation film 98 such as silicon dioxide film are damaged. Therefore, to increase the withstand voltage, the package is filled with sulfur hexafluoride gas which is extremely stable even at a high temperature of the order of 150° C. to prevent breakdown resulting from electric discharge.
Patent Reference 1: Japanese Patent No. 3395456
Patent Reference 2: Japanese Patent No. 3409507
Non-Patent Reference 1: Proceedings of 2001 International Symposium on Power Semiconductor Devices & IC's pp. 27 to 30